In the process of acquiring oil and/or gas from a well, it is often necessary to stimulate the flow of hydrocarbons via hydraulic fracturing. The term fracturing refers to the method of pumping a fluid into a well until the pressure increases to a level which is sufficient to fracture the subterranean geological formations containing the entrapped materials. This results in cracks and breaks that disrupt the underlying layer to allow the hydrocarbon product to be carried to the well bore at a significantly higher rate. Unless the pressure is maintained, the newly formed openings close. In order to open a path and maintain it, a propping agent or proppant is injected along with the hydraulic fluid to create the support needed to preserve the opening. As the fissure is formed, the proppants are delivered in a slurry where, upon release of the hydraulic pressure, the proppants form a pack or a prop that serves to hold open the fractures.
The behavior of the proppants in the fracturing fluids has to meet certain performance requirements. First, in order to place the proppants inside the fracture, proppant particles are suspended in a fluid that is then pumped to its subterranean destination. To prevent the particles from settling, a high viscosity fluid is often required to suspend them. The viscosity of the fluid is typically managed by addition of synthetic or natural based polymers. If the particles were more buoyant, a less viscous suspension fluid could be used, which would still convey the particles to the target area but which would be easier to pump into the formation. Second, it is desirable that the proppants remain where they are placed throughout the lifetime of the well after they have been injected into a fracture line. If changes within the reservoir during well production force the proppants out of position, production equipment can be damaged, and the conductivity of the reservoir formation can be decreased as its pores are plugged by the displaced proppants. Third, the proppants in the system should be resistant to closure stress once they are placed in the fracture. Closure stresses can range from 1700 psi in certain shale gas wells, up to and exceeding 15,000 psi for deep, high temperature wells. Care must be taken that the proppants do not fail under this stress, lest they be crushed into fine particles that can migrate to undesirable locations within the well, thereby affecting production. Desirably, a proppant should resist diagenesis during fracture treatment. The high pressures and temperatures combined with the chemicals used in fracturing (frac) fluids can adversely affect the proppant particles, resulting in their diagenesis, which can eventually produce fine particulate matter that can scale out and decrease the productivity of the well over time.
Current proppant systems endeavor to address these concerns, so that the proppants can be carried by the fracturing fluids, can remain in place once they arrive at their target destination, and can resist the closure stresses in the formation. One approach to preparing suitable proppants includes coating the proppant materials with resins. A resin-coated proppant can be either fully-cured or partially-cured. The fully-cured resin can provide crush resistance to the proppant substrate by helping to distribute stresses among the grain particles. A fully-cured resin can furthermore help reduce fine migration by encapsulating the proppant particle. If initially partially-cured, the resin may become fully cured once it is placed inside the fracture. This approach can yield the same benefits as the use of a resin that is fully-cured initially. Resins, though, can decrease the conductivity and permeability of the fracture, even as the proppants are holding it open. Also, resins can fail, so that their advantages are lost. Finally, resin-based systems tend to be expensive.
Another approach to preparing suitable proppants involves mixing additives with the proppant itself, such as fibers, elastomeric particles, and the like. The additives, though, can affect the rheological properties of the transport slurry, making it more difficult to deliver the proppants to the desired locations within the fracture. In addition, the use of additives can interfere with uniform placement of the proppant mixture into the fracture site. While there are known methods in the art for addressing the limitations of proppant systems, certain problems remain. There is thus a need in the art for improved proppant systems that allow precise placement, preserve fracture conductivity after placement, and protect well production efficiency and equipment life. It is further desirable that such improved systems be cost-effective.